Riot police fired tear gas and stun grenades at demonstrators on the fifth night of protests in Ferguson.

Mario Anzuoni

US president Barack Obama has called for peace on the streets of Ferguson, Missouri, after the fatal shooting of an unarmed black teenager by a police officer.

The president's call came as Missouri's governor moved to calm tensions in the mostly black suburb of St Louis by putting an African-American captain from the state's Highway Patrol in charge of the policing operation.

Protesters have been engaged in nightly stand-offs with heavily armed police since 18-year-old Michael Brown was fatally shot on Saturday during what authorities said was a struggle over a gun in a police car.

"Now is the time for healing. Now is the time for peace and calm on the streets of Ferguson," he told reporters on Martha's Vineyard, where he is on holiday with his family.

"Now is the time for an open and transparent process to see that justice is done."

Missouri authorities are withholding the name of the police officer who fatally shot the teenager, despite being pressured to release it.

Hacker group Anonymous has been threatening to release personal information about the police officer involved.

On Thursday the group identified a police officer by name in a Twitter post.

The St Louis County Police Department responded by issuing a denial on Twitter, suggesting that the group had identified the wrong person.

"Do not release more info on this random citizen," said a tweet from @stlcountypd.

A spokesman for the St Louis County prosecutor's office said the police officer's name would be released if he was charged.

The officer is currently on administrative leave.

Call for wider investigation

Mr Obama said he had asked the Department of Justice and the FBI to investigate the killing independently.

He has expressed concern to Missouri governor Jay Nixon about violence in the wake of the shooting and has asked attorney-general Eric Holder and the US attorney on the scene to report back to him about the investigation.

"We lost a young man, Michael Brown, in heartbreaking and tragic circumstances. He was 18 years old and his family will never hold Michael in their arms again," Mr Obama said.

"When something like this happens, the local authorities, including the police, have a responsibility to be open and transparent about how they are investigating that death and how they are protecting the people in their communities."

Mr Obama said there was never an excuse for looting or violence against police.

But he also said there was no excuse for police to use excessive force against peaceful protesters or to throw people, including journalists, in jail for exercising their rights under the US constitution.

"We all need to hold ourselves to a high standard," Mr Obama said.

The Missouri incident recalls the US president's emotional reaction to the shooting of Trayvon Martin, the black teenager who was shot to death by a white Hispanic neighbourhood watch volunteer in Florida in 2012.